A device for forming at least part of a head part of a liquid treatment apparatus is arranged to receive a connecting head of a replaceable liquid treatment cartridge and includes at least one ratio-setting valve. The ratio-setting valve is provided with at least two first ports, each placeable in essentially sealed liquid communication with respective cartridge ports of a connecting head of a replaceable liquid treatment cartridge inserted into the device, and with at least one second port. The ratio-setting valve includes at least one movable regulating part for setting a volumetric flow rate ratio between a first flow of liquid between a first of the first ports and the at least one second port and a second flow of liquid between a second of the first ports and the at least one second port at any of a plurality of values.